1. Field of the Invention
This invention relates to improvements in burning pulverous or granular materials. More particularly it pertains to such improvements as applied to burning cement raw meal to cement clinker.
2. Description of the Prior Art
Many plants for burning or sintering of ores, lime and cement raw materials have been developed in recent years and are of well known design. Frequently the suspension preheater consists of a number of cyclones through which the raw material is passed in succession from the inlet to the outlet of the preheater while exchanging heat with the waste gases from the rotary kiln passing in the opposite direction through the preheater to a dust precipitator. A typical example of such a dust precipitator is an electrostatic precipitator.
The cooler, which generally forms an integral part of the rotary kiln and thus rotates with the kiln, usually comprises a number of cooler tubes mounted in planetary fashion around the kiln. The material burnt in the rotary kiln is passed through chutes into the cooler tubes in which it is cooled by means of atmospheric air drawn through the cooler tubes in countercurrent flow to the hot material.
By the action of heat exchange in the planetary cooler between atmospheric air and the hot clinker the air is brought to a rather high temperature (of the order of 800.degree. to 900.degree.C) so that the heated cooling air has a high heat content which it is important to recover. This is preferably done by using the heated cooling air in the preheating and burning process, thereby improving the overall heat economy of the plant. However, it is not always possible to correlate the amount of cooling air needed to cool the burnt material to an acceptable temperature for handling after discharge from the cooler with the amount of combustion air necessary for the burning process performed in the rotary kiln.
In plants having stationary coolers such as grate coolers it is well known to use part of the heated cooling air in the preheater for recovering part of the heat. However, in planetary coolers associated with a rotary kiln it is a complicated matter to branch off part of the cooling air and still ensure sufficient cooling of the hot material. I have invented a method and plant for utilizing the heat of the preheated cooling air in such an improved manner as to improve the efficiency of such plants.